I have read on these forums to fill up your fuel tank till you can see diesel in the neck of the fill up tube. I don't have the harpoon mod on my truck so this took a while. I couldn't see diesel in the neck (foam, but it kept receding out of site) but all of a sudden I saw diesel leaking from the top of the tank (near the front) from under my truck. Has this happened to anyone before? I couldn't see exactly the place it was leaking from and I didn't want diesel all over me so I didn't look to closely. It's a stock 29 gallon tank.

 

There are several places that it might be leaking when filled to the top. There is a vent tube front and rear, the access hole bung nut, the filler spout, and the vent tube. Doesn't matter too much, because you have to get to the top of the tank to fix any one of those.

You can see some photos of the top of the tank here:
Powerstroke Fuel Tank Modification

If the vent hose just popped off, it might be possible to reach in there and replace it without dropping the tank or lifting the bed.

 

Thank you for the reply! I figured there are vent tubes somewhere on the tank. It was at the front of the fuel tank where it was leaking.

 

I have filled it up like this before and not leaked. It seems like I can get an extra 3 to 4 gallons on top of the 29 gallon capacity, which is nice.

 

Re-reading my post above I see that it may be confusing with regard to vents.

There are two small vent tubes, one in front and one in back, which are intended to let air into the tank as fuel is consumed. Those are thin wall rubber hoses that just push on by hand. They may easily pull off, and they can also easily get cut or torn.

Then there is also a filler tube vent hose. It lets vapor escape while you are pumping fuel into the tank. It has hose clamps.

 

Thank you again, rep points your way! Also, it was leaking on the drivers side and not on the passenger side so I will check the hose connections that I saw in your picture.